PMBOK 5th Ed, p67 says the project should be written by the sponsoring entity and recommends the project manager to participate in writing that charter. So, if you think about it, you've got the sponsor who has this grand vision of building some end-product or service. You need one charter that outlines this high level vision. Now, you as the project manger, assigned to accomplishing this vision, will have to break up the work into phases. The Charter, again, is high level, so you only need one.

Now, stepping out of the PMP exam world and into reality, if you're treating each phase as its own project with unique deliverables, risks, acceptance requirements, etc, you'd probably create unique charters at your level to capture relationships with stakeholders and team members.

You should have only one project charter per project. Think back to the main purpose of the project charter - in light of that, multiple project charters are redundant and defeat the purpose. If it is known that there will be multiple phases of the project then this COULD be mentioned very briefly in the project charter but ideally it would be in the project plan and other artifacts that come after the project charter.

I haven't seen a project with multiple charters even though I have had multiple projects that span multiple phases.